The web was trapped in the early Cretaceous period as sticky sap seeped from a tree in what is now Spain. It had hung from a tree so that it would catch insects on the wing.

The sap may have dripped onto the web, or the web may have blown onto its surface, says David Grimaldi of the American Museum of Natural History in New York, US. Then more sap covered it, forming a small amber "stalactite" 18 millimetres long and 7.5 millimetres wide.

Palaeontologists who found the amber sliced it into three thin sections, revealing at least 26 silk strands, some interconnected. The web is not complete enough to be firmly identified as an orb web, with a spiral strand wound on radial spokes. But Grimaldi says the fragments are consistent with orb webs. "It's a geometrically complex web, certainly not a random assortment of strands like a cobweb. It was certainly in one plane," he told New Scientist.

Ancient spinners

Spiders are far more likely to be fossilised than their silk and such finds have shown that spiders have had spinnerets for extruding silk for at least 400 million years. The oldest known spider silk was found in 2003 in 130-million-year-old amber from Lebanon, but it was only a single 4-millimetre strand (see Silken clue to ancient spider's mastery), revealing very little about the web it came from.

The new fossil is by far the oldest web fragment. It contains a group of five strands in the same plane, at least three of which are connected to a perpendicular incomplete strand in the same way as in modern orb webs.

The amber also contains bits of prey stuck to the web, including a mite, a fly, a beetle and a wasp. "They are basically the type and size of prey you expect in a web several centimetres in size" today, although the fossil species are all extinct, Grimaldi said.

That means similar webs have shaped the evolution of flying insects for over a hundred million years. For example, moths and butterflies, which evolved at the same time as flowering plants about 130 million years ago, have scaly bodies that allow them to escape from sticky webs.

Journal reference: Science (vol 312, p 1761)

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The location of the spider silk strands have been drawn on this image. Magnified images of a mite and bubbles attached to a strand are shown in boxes. The scale bar is 1 millimetre long. (Image: Science)